I tried switching to the sugar-free version once, thought it would be better for my teeth and figure. But I noticed something funny: the drink started tasting kind of chemical in my mouth, like my tongue was slightly going numb. Then I read up on it - it's sucralose, and it really does give that effect for some people. Anyway, I went back to the regular one, but now I drink it less often and in smaller amounts.
The main catch that nobody mentioned: artificial sweeteners can trick your body. Your brain expects calories from the sweet taste, but doesn't get them - that can make you eat more of other stuff later, trying to "close" that gap. So the calorie savings might be an illusion. Plus, if you drink energy drinks often (sugar-free or not), the caffeine still puts a strain on your heart and nervous system - it's the same problem in both versions.
If you're choosing between them - sugar-free is less damaging for your teeth, that's a fact. But is it healthier overall? No, it's more like choosing the lesser evil. And really, it's better to look for alternatives - regular coffee, tea, water with lemon. Energy drinks are energy drinks, but don't fool yourself into thinking sugar-free is already a healthy drink.